The present invention relates to a process for obtaining polymers based on vinyl chloride which are capable of giving plastisols having specific properties. More particularly, the invention relates to a process of seeded microsuspension polymerisation of vinyl chloride with or without one or more copolymerisable monomers.
It is known to use plastisols prepared from polymers based on vinyl chloride in the manufacture of various articles, such as floor and wall coatings, plastified coated fabrics, and mastics.
In view of the vigorous competition, many studies have been carried out for the purpose either of attempting to find new applications for these plastisols or of improving their properties.
A further subject of numerous studies has been processes for manufacturing polymers based on vinyl chloride which are capable of giving plastisols having specific properties, thereby opening up the way to new applications.
Japanese Patent Application JP 62-231594 discloses a process for manufacturing polymers based on vinyl chloride which are capable of giving plastisols having low viscosity, good thermal stability and good resistance to water absorption. The essential feature of this process is the presence of hydrotalcite, which is introduced into the reaction medium before or during the initial phase of polymerisation of the vinyl chloride.
Moreover, a process for preparing vinyl chloride polymers which is capable of giving plastisols having a low initial viscosity, stability in the viscosity in the course of storage, and good degassing properties has been described in French Patent FR 2163573. That document teaches working in an aqueous medium under the action of organosoluble initiators and water-soluble reducing agents, in the presence of alkali metal salts of higher fatty acids containing 8 to 18 carbon atoms and at least one compound selected from the group (A) consisting of higher alcohols and higher fatty acids having 8 to 18 carbon atoms.
Comparative Example 5 of the document FR 2163573 shows that a polymerisation of vinyl chloride conducted in the absence of a compound from group (A) leads to highly viscous plastisols which are poorly suited to degassing and have poor transparency.
Apart from the documents mentioned above, the literature provides no processes for obtaining plastisols having a set of appropriate properties, given the difficulty of improving simultaneously two or more of their properties. Thus the resistance to water absorption is often sacrified for the benefit of thermal stability, or else good suitability to degassing at the expense of viscosity.
One aim of the present invention is to provide plastisols which lend themselves well to debubbling, or plastisols containing no air bubbles after kneading, while retaining or improving at least one of the following properties : thermal stability, resistance to discoloration, UV stability, transparency, rheology and hydrophobicity. The target plastisols are in particular those which lend themselves well to debubbling and have good thermal stability, those which lend themselves well to debubbling and have adequate hydrophobicity, and those which lend themselves well to debubbling and have good thermal stability and adequate hydrophobicity.
This aim is achieved in accordance with the present invention by a process of seeded microsuspension polymerisation of vinyl chloride with or without one or more polimerizable comonomers in an aqueous medium under the action of at least one organosoluble initiator, of at least one reducing agent selected from the group (S) consisting of alkali metal sulphoxylates, sulphites and metabisulphites, of at least one water-soluble metal salt selected from the group (M) consisting of salts of iron, copper, cobalt, nickel, zinc, titanium, vandium, manganese, chromium, cerium, of tin and of silver, in the presence of at least one alkali metal or ammonium salt of a fatty acid containing 8 to 22 carbon atoms.
The present invention firstly provides a process for obtaining latex containing polymer particles based on vinyl chloride, which consists in polymerising the corresponding monomer(s) in microsuspension in the presence of water, at least one water-soluble metal salt selected from the group (M), at least one alkali metal or ammonium salt of a fatty acid containing 8 to 22 carbon atoms, at least one reducing agent selected from the group (S), at least one seed polymer (P1) whose particles contain at least one organosoluble initiator and, if desired, at least one seed polymer (P2).
Microsuspension polymerisation, or polymerisation in microsuspension, means the polymerisation in the presence of organosoluble initiators of at least one monomer which is dispersed by means of mechanical energy in an aqueous medium containing an emulsifier as stabiliser to give a dispersion of particles whose average diameter is less than 5 xcexcm.
Polymers based on vinyl chloride are the homopolymers and the copolymers, the latter containing at least 50% by weight of vinyl chloride and at least one monomer copolymerisable with vinyl chloride. The copolymerisable monomers are those generally employed in conventional techniques of copolymerising vinyl chloride. Mention may be made of vinyl esters of mono- and polycarboxylic acids, such as vinyl acetate, propionate and benzoate; unsaturated mono- and polycarboxylic acids, such as acrylic, methacrylic, maleic, fumaric and itaconic acid and their aliphatic, cycloaliphatic and aromatic esters, their amides, and their nitrites; vinyl halides and vinylidene halides; alkyl vinyl ethers; and olefins.
Preparation of the Seed Polymer (P1)
The seed polymer (P1) required for the polymerisation in accordance with the invention is prepared in accordance with conventional techniques of microsuspension polymerisation. It is present in the form of a dispersion of polymer particles whose average diameter is between 0.2 and 2 xcexcm and, preferably, between 0.3 and 1 xcexcm.
One means of preparing the polymer (P1) consists in employing water, vinyl chloride alone or in combination with one or more copolymerisable monomers, an organosoluble initiator and an anionic emulsifier alone or in combination with a non ionic emulsifier. The monomer or monomers are finely dispersed in water with the aid of a means of mechanical energy such as, for example, a colloid mill, high-speed pump, vibratory stirrer, ultrasound apparatus or high-pressure mixer disperser. The microsuspension obtained is then heated under autogenous pressure and with moderate stirring at a temperature which is generally between 30 and 65xc2x0 C. Following the drop in pressure, the reaction is stopped and the unconverted monomer or monomers are outgassed.
The organosoluble initiators to be employed in the preparation of the polymer (P1) are represented by organic peroxides, such as diacyl peroxides (for example lauroyl, decanoyl or caproyl peroxides), tert-butyl diethylperacetate, dicetylhexyl percarbonate, diacetyl peroxide and dicetyl peroxocarbonate. The preferred initiators are those which are also insoluble in water.
The organosoluble initiator is selected such that its deployment in a reasonable amount makes it possible to obtain polymers (P1) containing between 1 and 10% by weight of this initiator for a preparation period of between 4 and 16 hours.
The proportion by mass of organosoluble initiator present in the particles of the polymer (P1) is advantageously between 1 and 4%.
The preferred organosoluble initiator is lauroyl peroxide.
Preparation of the Seed Polymer (P2)
The optional seed polymer (P2) is present in the form of a dispersion of polymer particles based on vinyl chloride whose average diameter is between 0.05 and 0.5 xcexcm, and preferably between 0.08 and 0.2 xcexcm.
This particle dispersion can be obtained by conventional techniques of emulsion polymerisation.
One means of preparing the polymer (P2) consists in employing water, vinyl chloride alone or in combination with a copolymerisable monomer, a water-soluble initiator and an anionic emulsifier with or without a non-ionic emulsifier.
The reaction mixture is heated under autogenous pressure and with moderate stirring at a temperature of between 30 and 65xc2x0 C. Following the drop in pressure, the reaction is stopped and the unconverted monomer or monomers are degassed.
The water-soluble initiators required for the preparation of the polymer (P2) are generally represented by hydrogen peroxide, alkali metal or ammonium persulphates alone or together with water-soluble reductants, such as alkali metal sulphites or bisulphites. The amounts use, which vary greatly, depend on the initiator system selected and are adjusted so as to ensure polymerisation within times of between 4 and 12 hours.
Preparation of the Latex
In the process according to the invention, the amount of reducing agent employed is less than 0.1% by weight and, preferably, is between 30 and 200 ppm relative to the monomer(s) deployed.
The reducing agent which is advantageously selected is sodium or potassium metabisulphite.
Generally speaking, the reducing agent is used in the form of an aqueous solution with a concentration of between 1 and 100 g/l. A concentration of between 1 and 20 g/l is preferred.
Preferably, the introduction of this aqueous solution into the reaction medium is begun before the temperature of the said medium reaches the target polymerisation temperature, and is continued during the polymerisation at a rate which can be adjusted as a function of the cooling capacity of the reactor.
The reducing agent(s) selected from the group (S) can be used together with conventional reducing agents, i.e. those commonly used in microsuspension, such as, in particular, alkylphosphoric acids, lactones, ketones, carbazQnes, mono- or polycarboxylic acids, especially ascorbic acid, and derivatives thereof.
Among the conventional reducing agents, ascorbic acid is advantageously selected.
In accordance with the present invention, the amount of fatty acid alkali metal or ammonium salt employed is generally between 0.3 and 3% and, preferably, between 0.5 and 1.5% by weight relative to the monomer(s) deployed.
The carbon chain of the fatty acid can be linear or branched. It can be saturated or may include one or more unsaturated bonds. It may also include one or more hydroxyl or epoxy functions.
The preferred fatty acid ammonium or alkali metal salts are those containing 12 to 22 carbon atoms, such as, for example, the salts of lauric acid, myristic acid, palmitic acid or stearic acid.
The fatty acid ammonium salts containing 12 to 22 carbon atoms, or mixtures thereof, have proven to be of great interest.
The fatty acid salts can be employed in solid form, in suspension or in solution. However, it is preferred to use them in aqueous solution, even if dissolving them necessitates a temperature higher than the ambient temperature.
In accordance with the present invention, the fatty acid ammonium or alkali metal salts can be introduced into the reaction medium before and/or during and/or after the polymerisation. These fatty acid salts are advantageously introduced during the polymerisation.
The fatty acid salts can be formed in situ, by first introducing the fatty acid into the reaction medium and by injecting a base during the polymerisation until the fatty acid is completely neutralised.
In the process of the invention, the water-soluble metal salt selected from the group (M) is employed in an amount such that the mass ratio of metal salt to monomer(s) deployed is between 0.01 and 100 ppm, preferably between 0.05 and 10 ppm and, advantageously, between 0.1 and 5 ppm. Water-soluble copper salts are particularly preferred.
The water-soluble metal salt can be introduced before and/or during the polymerisation. It is preferably introduced before the polymerisation.
The total amount of polymers (P1) to be employed is between 0.5 and 10% by weight and, preferably, between 1 and 5% by weight relative to the monomer(s) deployed.
When working in the presence of at least one polymer (P2), the total amount of polymers (P2) to be employed is between 0.1 and 10% by weight and, preferably, between 0.5 and 5% by weight relative to the monomer(s) deployed.
In one preferred embodiment of the invention, both a polymer (P1) and a polymer (P2) are employed, the particles of (P2) having an average diameter smaller than that of the particles of (P1).
The amount of water required for the process according to the invention must be such that the initial concentration of seed polymers plus the monomer(s) deployed is between 20 and 80% and, preferably, between 45 and 75% by weight relative to the reaction mixture.
Although unnecessary, it is possible to add at least one anionic emulsifier to the reaction medium before and/or during and/or after the polymerisation, the said emulsifier being selected preferably from the group consisting of alkyl sulphates, alkylsulphonates, vinylsulphonates, arylsulphonates, alkylsulphosuccinates and alkali metal alkyl phosphates.
According to the invention, the reaction medium is heated under autogenous pressure at a temperature which is generally between 35 and 70xc2x0 C. and, preferably, between 45 and 60xc2x0 C.
The polymers based on vinyl chloride which are prepared by the process of the invention are separated from the polymerisation medium by any known means, such as filtration, coagulation/suction filtration, flaking, centrifugal decanting and drying. Separation is preferably carried out by drying and the polymers based on vinyl chloride are advantageously dried by atomisation.
The invention secondly provides the vinyl chloride-based polymer thus obtained which is suitable for the preparation of plastisols having specific properties.
The invention thirdly provides the preparation of these plastisols by mixing the vinyl chloride-based polymer thus obtained with a plasticizer. The invention additionally provides the plastisols thus prepared.
Ixe2x80x94Preparation of the Seed Polymer (P1)
The following constituents (by weight) are introduced in succession into a reactor which is stirred at 35 rpm and regulated to 15xc2x0 C.
120 parts of water,
0.14 part of monopotassium phosphate,
0.02 part of sodium hydroxide,
3.4xc3x9710xe2x88x923 parts of paraquinone powder,
1.88 parts of lauroyl peroxide,
100 parts of vinyl chloride,
and 1.5 parts of sodium dodecylbenzenesulphonate in the form of a 10% aqueous solution,
the reactor being placed under vacuum just before introducing the vinyl chloride.
The vinyl chloride is subsequently dispersed finely in the aqueous medium at a temperature of less than or equal to 35xc2x0 C., by stirring the said medium at 5500 rpm for 105 minutes. The reaction medium is then brought to the target polymerisation temperature of 45xc2x0 C. under autogenous pressure, while stirring at a rate of 30 rpm. The paraquinone is then introduced continuously at a constant rate of 0.033 part by weight/h.
After the drop in pressure to a value of 3.5 bars, the unreacted vinyl chloride is outgassed.
IIxe2x80x94Preparation of the Seed Polymer (P2)
The following components (by weight) are introduced into a reactor fitted with a stirrer 138 parts of water, an aqueous solution containing 0.5 part of lauric acid and 0.15 part of sodium hydroxide.
The reactor is subsequently placed under vacuum prior to the introduction of 100 parts of vinyl chloride. The reaction medium is then brought to the target temperature of 58xc2x0 C.
When the medium reaches 45xc2x0 C., an aqueous solution containing 0.05 part of potassium persulphate per 14 parts of water is introduced continuously. Two hours after the beginning of introduction of this solution containing 1.3 parts of sodium dodecylbenzenesulphonate is added continuously at a constant rate for 8 hours.
When the internal pressure is 4 bars, the unreacted vinyl chloride is recovered under vacuum and then the reactor is aerated and then cooled.